I've gotten to play with the 360 quite a bit over the past two weeks, and I'd just like to say that Achievements are a wonderful, maddening, and ingenious addition to the console. In fact, the whole Gamertag concept is beautifully executed.
Some of you will look at the above and have no idea what I'm going on about. Let me clarify.
Gamertags are the new foundation of Xbox Live, Microsoft's online community. They're user profiles that, along with a simple reputation economy, are used to match you up with other players for online play. In theory, they also try to group players that are just "here for fun" together, while putting the players that are here to "roxxor u all" somewhere else. From my limited online experiences, it seems to work well - but that's not really the interesting part.
Achievements are the more interesting facet. They're collectible 'awards' for your Gamertag that relate to reaching smaller goals within a game. For example, while playing Gun on the 360, you get a new Achievement every time you complete a section of the main story arc. We've all seen this before on a game level - unlockable goals are nothing new - but the genius of the 360 implementation is that it makes these goals universal, persistent, and publicly viewable.
The last one might be the key - by letting everyone see other profiles, Microsoft has opened a window to any user that plugs their console into the net. All the users, even the ones that only play single-player games, are now playing in the same 'space.' Last week, on the MUD-DEV mailing list, Damion Schubert posted a summary of his theory that "there is a huge group of people who don't play MMOs to group with other people, they play MMOs because other people are there."
Could the constant reinforcement brought by these Achievements, and the natural desire to compare yourself to others, lend some of the addictive qualities that MMOs contain to an entire console platform?